The present invention relates to a method of printing a substrate using an inkjet printer, which printer includes a holder which rotatably receives a roll on which the substrate is wound, a downstream print zone and an inkjet printhead for printing the substrate in the print zone, a transport means for engaging and transporting the substrate to the print zone as the substrate is unwound from the roll, and a guide element which is situated downstream of the roll in front of the transport means to guide the substrate from the roll to the transport means. The printing method includes the steps of transporting the substrate over a predetermined distance with control of the transport means, during which transport the guide element is moved from a first position occupied by said guide element prior to the transport, to a second position such that the distance over which the substrate extends between the roll and the transport means is smaller as a result of the movement and after the substrate has been transported over the predetermined distance; printing a strip of the substrate with control of the inkjet printhead, and, after printing of the strip, the re-transport of the substrate over the predetermined distance during which the guide element is moved, which transport is followed by printing a following strip of the substrate.
The present method is used inter alia to prevent damage to the substrate during its transport. The transport means, which is frequently a transport nip which engages the substrate at a number of places distributed over the width of the substrate, has a lower mass inertia than the roll on which the substrate is wound, at least when there is a specific minimum amount of substrate present on the roll. The transport of the substrate and its simultaneous unwinding from the roll can, in this case take place at an adequate speed if the transport means is, for example, driven by a very powerful motor, both the transport means and the roll being accelerated. The result of using such a powerful motor is that considerable forces are exerted on the substrate, with the risk that the substrate will tear. In addition, a powerful motor of this kind has the disadvantage that it is less suitable for very accurate control. In inkjet printers in particular, accurate control of the transport means is very important, because printing often takes place by printing the substrate in a number of swaths, and in each swath, part of the substrate is printed, often a strip of the same width as the inkjet printhead. All the sub-images together form the image for printing. For accurate juxtaposition of the sub-images, accurate transport of the substrate is desirable.
It has been determined to provide the transport means with a low-power driving motor and to dispose a movable guide element between the roll and the transport means. By moving this element, it is possible to reduce the distance over which the substrate extends between the roll and the transport means. This prevents any sudden increase in substrate tension. The motor that drives the transport means consequently does not have to move the entire roll each time. It is thus possible to use a relatively low power drive for the transport means, it being a relatively simple matter to make such drive accurate. In this method, the roll can be driven separately with a powerful but less accurate motor.
A disadvantage of this method is that for the accurate transport of the substrate by the transport means there is required not only an accurate drive but also a very accurate transport means and guide element. The mechanical tolerances such as roundness, straightness, mutual parallelism, etc., are subject to stringent requirements in order that the transport may take place with the required accuracy. In addition, even with very close tolerances, there are still a relatively large number of irregularly recurring juxtaposition faults between the sub-images. These juxtaposition faults are indications of random faults in the substrate transport.